In recent years, a rapidly increasing number of electronic devices have become portable and cordless, and as power sources for operating these devices, a growing number of non-aqueous electrolyte secondary batteries with a high voltage and a high energy density have come into practical use. As a positive electrode of the non-aqueous electrolyte secondary batteries, a composite oxide of a transition metal and lithium, having a high redox potential, is generally used. As the composite oxide used are lithium cobaltate, lithium nickelate, lithium manganate and the like, as well as an oxide containing plural transition metals. As a negative electrode of the non-aqueous electrolyte secondary batteries, on the other hand, a carbon material is commonly used.
There lies a problem in the non-aqueous electrolyte secondary batteries that a battery capacity gradually decreases with repeated charge/discharge cycles. It is considered as one of the causes of this problem that a transition metal contained in a composite oxide constituting a positive electrode elutes into a non-aqueous electrolyte to deposit on the negative electrode. When a lithium ion secondary battery using lithium cobaltate for the positive electrode is stood still at a high potential of 4.2 V or higher, for example, cobalt in lithium cobaltate is known to elute in the form of cobalt ions. The elution of cobalt from the positive electrode causes destruction of the crystal structure of lithium cobaltate, thereby decreasing a positive electrode capacity. Further, there may be a case where a very small degree of short circuit occurs between the positive electrode and the negative electrode when the cobalt ions having eluted deposit on the negative electrode and the deposited matter goes on growing. Moreover, cobalt having deposited on the negative electrode functions as a catalyst for proceeding decomposition of the non-aqueous electrolyte, leading to gas generation within the battery. Such a problem is particularly conspicuous when a battery in a charged state is placed under a high-temperature atmosphere at 45° C. or higher.
Under such circumstances, Japanese Laid-Open patent publication No. Hei 11-67211 has described that an antioxidant contained in a positive electrode makes it possible to prevent deterioration in charge/discharge cycle characteristic caused by deterioration in positive electrode. Further, Japanese Laid-Open patent publication No. Hei 1-167965 has described that it is effective for a non-aqueous electrolyte to contain an antioxidant. In Japanese Laid-Open patent publication No. Hei 10-247517, on the other hand, an antioxidant is contained in a non-aqueous electrolyte for the purpose of suppressing oxidative decomposition of the non-aqueous electrolyte. Further, an antioxidant has been generally used as an additive in a separator interposed between the positive electrode and the negative electrode of the non-aqueous electrolyte secondary batteries. The reason for the use of the antioxidant in the separator is that the separator is comprised of a polyolefin resin which is relatively vulnerable to oxidation when the resin contacts with the air and is heated.
However, there have also been made reports that addition of an antioxidant into a battery causes a decrease in battery performance. For example, Japanese Laid-Open patent publication No. Hei 12-30685 has described that it is effective to use a separator from which an antioxidant elutes to propylene carbonate in an amount of not more than 2,000 ppm in order to minimize an adverse effect which the antioxidant exerts on battery performance, and that with concentration of the antioxidant in propylene carbonate exceeding 2,000 ppm, the battery performance deteriorates significantly. Further, Japanese Laid-Open patent publication No. Hei 12-251943 has described that the use of a phenolic antioxidant in a separator particularly causes a remarkable decrease in battery capacity at high temperatures.
For the separator comprising a polyolefin resin used is a non-woven fabric or a microporous film with a single-layered or multi-layered structure prepared by extraction, drawing, melt blowing, or the like. Particularly in general use has been a microporous film separator with a structure of a single layer or double layers with a thickness of not more than 30 μm, comprising a polyethylene resin which can be made thinner with ease. Further, the separator comprising a polypropylene resin has been used for the battery together with an additive such as an antioxidant or a stabilizer, since the separator has a difficulty in becoming thinner and tends to be oxidized in terms of the physical properties of the polypropylene resin. For example, Japanese Laid-Open publication No. 2000-204174 has described that a sulfuric antioxidant with an oxidation potential of +4.5 V or higher with respect to lithium is effective and it will neither decompose nor be denatured in the range of a working voltage of the lithium secondary battery.
Despite the disclosures of the various techniques using the antioxidants as thus described, the drawback of the antioxidant that it deteriorates battery performance is conspicuous because the structure of the separator, the physical properties of the antioxidant to be used and the arrangement of the separator and the electrode plate are inappropriate while an excellent function inherent in the antioxidant has not been sufficiently brought out.